Not Applicable
Not Applicable
1. Technical Field
This invention relates to electronic article surveillance (EAS) systems, and more particularly to noise reduction in EAS receivers.
2. Description of the Related Art
EAS systems are typically used to prevent unauthorized removal of articles from a protected area. EAS tags are attached to articles designated for protection, and when active, the EAS tags will trigger an action, such as setting off an alarm, if carried through an EAS interrogation zone. EAS interrogation zones are typically positioned at the exits of the protected area. For authorized removal of an article, the attached EAS tag is removed or deactivated so the article can be carried through the interrogation zone and removed from the protected area without triggering the EAS system. EAS markers, labels, and tags are used interchangeably herein and refer to markers, labels, tags, and the like that trigger EAS systems.
There are several types of EAS systems presently in use including RF, microwave, harmonic, and acoustomagnetic or magnetomechanical. Magnetomechanical EAS systems are used herein to describe one embodiment of the invention, but can be applied to other types of EAS systems within the scope of this disclosure. A magnetomechanical EAS marker is typically made of a xe2x80x9cresonatorxe2x80x9d, an elongated strip of magnetostrictive ferromagnetic material, disposed adjacent a xe2x80x9cbiasxe2x80x9d, a ferromagnetic element that, when magnetized, magnetically biases the strip and arms it to resonate mechanically at a preselected resonant frequency. The marker resonates when subjected to an electromagnetic interrogation field at a frequency at or near the marker""s resonant frequency. The response of the marker can be detected by an EAS receiver, which can trigger an alarm. A complete description of a magnetomechanical EAS system is given in U.S. Pat. No. 4,510,489.
Common electronic devices such as flourescent lights, computer monitors, power control circuitry, and other electronic noise sources may interfere with EAS receivers. These noise sources increase the noise level at the EAS system""s receive antenna causing a reduction in sensitivity. The reduction in sensitivity makes it more difficult to detect EAS tags that are far from the receive antenna, and makes desirable wide antenna separation to accommodate wide exits difficult to achieve.
Retailers are demanding wide separations between receive antennas, which requires that the detection algorithm be very sensitive as it must detect a tag far away from the receive antenna. In electrically noisy environments this is not always possible because the noise source may overlap the tag signal in both time and frequency, making, separation of the tag and noise signals difficult or impossible using only the receive antenna input.
One solution to the noise problem has been to change the antenna pattern so that there is a null in the direction of the interference source. Summing multiple sub-antennas (coils) within the main antenna in such a way as to produce the desired antenna pattern typically does this. The procedure involves manual optimization, which will not automatically adjust to changing environments, and does not always produce a working solution. The altered antenna pattern will generally affect tag sensitivity making tag detection more difficult.
An alternate solution involves adding a reference antenna together with a manually adjusted hardware-coupling network. The reference antenna is spatially separated from the main receive antenna in such a way that the reference antenna senses the interference signal but does not sense the tag signal. The two antenna inputs can then be combined using a coupling network in such a way that the noise is effectively canceled. The coupling network typically is tuned to match the noise source and environment. This procedure also involves manual optimization and will not automatically adjust to changing environments. The required hardware-coupling network may match gain and phase at only one or more frequencies, and will not easily work as a general primary vs. reference channel equalizer, as is desired.
The use of a reference antenna may make it possible to regain some of the system sensitivity lost to noise sources, however, because of the nature of a typical retailer""s environment the noise sources may be turned on and off and/or moved periodically. This requires a service call for the coupling network to be manually retuned to restore system sensitivity. Even worse, some noise sources change during the day, so that full system performance is never restored. A network that regains lost sensitivity due to noise, and automatically tunes itself is desirable.
An EAS receiver, and corresponding method, is provided that includes a primary antenna for receiving a first signal, which includes an EAS tag signal and an interference noise signal. The primary antenna is coupled to a primary channel for amplifying and filtering the first signal. A reference antenna is used for receiving a second signal, which includes the interference noise signal. The reference antenna is coupled to a reference channel for amplifying and filtering the second signal. An adaptive filter is connected to the reference channel output. The adaptive filter is responsive to an update algorithm. The output of the adaptive filter is approximately equal to the interference noise signal. A summing network is connected to the adaptive filter output and to the primary channel output. The output of the adaptive filter is subtracted from the output of the primary channel. The resulting output of the summing network is approximately equal to the EAS tag signal. The adaptive filter can be updated according to the output of the reference channel and to an error signal from the summing network.
A detection filter can be connected to the summing network to detect a valid EAS tag signal. A sample and hold circuit is connected to the detection filter for sampling and holding the EAS tag signal. A threshold comparison is performed between the sampled and held EAS tag signal and a selected threshold value. An output signal is provided to indicate whether the sampled and held EAS tag signal is greater than the selected threshold value indicating an EAS tag has been detected.
In a second aspect of the invention, an EAS receiver is provided that includes a primary antenna for receiving a first signal (z), which includes an EAS tag signal (x) and an interference noise signal (y) wherein z=x+y. The primary antenna is coupled to a primary channel for amplifying and filtering the first signal wherein the amplified and filtered first signal equals z1=x1+y1. The primary channel has a transfer function H1. A reference antenna is used for receiving a second signal, which includes the interference noise signal (y). The reference antenna is coupled to a reference channel for amplifying and filtering the second signal wherein the amplified and filtered second signal equals (y2). The reference channel has a transfer function H2. An adaptive filter having a transfer function W=(H2)xe2x88x921xc2x7H1 is connected to the reference channel. The adaptive filter is responsive to an update algorithm. An output of the adaptive filter (yW) is approximately equal to the interference noise signal (y1), wherein yw=Wxc2x7y2≈y1. A summing network is connected to the adaptive filter output and to the primary channel output. The output of the adaptive filter (yw) is subtracted from the output of the primary channel (z1), wherein the output of the summing network is approximately equal to the EAS tag signal (x1), wherein e=z1xe2x88x92yw=x1+y1xe2x88x92yw≈x1+y1xe2x88x92y1≈x1.
The update algorithm is updated according to a filter update algorithm W(n+1)=W(n)+xcexcxc2x7y2xc2x7e, where (n) refers to the nth iteration of receiver processing and xcexc is a gain constant affecting the tracking bandwidth.
In a third aspect of the invention, an EAS system responsive to the presence of an EAS tag within an interrogation zone is provided that includes an interrogation zone formed by a transmitter and antenna that generates an electromagnetic field within the interrogation zone. A receiver and receive antenna for detecting an EAS tag disposed in said interrogation zone. The receiver made as described herein, including a primary antenna for receiving a first signal, which includes an EAS tag signal and an interference noise signal. The primary antenna is coupled to a primary channel for amplifying and filtering the first signal. A reference antenna is used for receiving a second signal, which includes the interference noise signal. The reference antenna is coupled to a reference channel for amplifying and filtering the second signal. An adaptive filter is connected to the reference channel output. The adaptive filter is responsive to an update algorithm. The output of the adaptive filter is approximately equal to the interference noise signal. A summing network is connected to the adaptive filter output and to the primary channel output. The output of the adaptive filter is subtracted from the output of the primary channel. The resulting output of the summing network is approximately equal to the EAS tag signal if one is present.
Objectives, advantages, and applications of the present invention will be made apparent by the following detailed description of embodiments of the invention.